1. Field of the Invention
The present invention relates to a liquid state detection sensor for detecting the temperature and the concentration of a particular component of a liquid contained in a liquid container.
2. Description of the Related Art
In recent years, NOx selective catalytic reduction (SCR) has come to be employed in exhaust emission control apparatus which reduce nitrogen oxides (NOx) emitted from diesel vehicles, for example, into harmless gases. A urea aqueous solution is used as a reducing agent for that purpose. It is known that the use of a urea aqueous solution having a urea concentration of 32.5 wt % is appropriate for carrying out efficient chemical reduction. However, a urea aqueous solution contained in a urea solution tank installed in an automobile is stored under severe environmental conditions and its urea concentration may vary with age, for example. Further, another kind of liquid (e.g., light oil) or water may erroneously be poured into the urea solution tank. Therefore, so as, to manage the urea concentration of a urea aqueous solution, the urea concentration is detected by attaching a urea concentration sensor to the urea solution tank.
Incidentally, the thermal conductivity of a urea aqueous solution depends on its urea concentration. Therefore, a concentration sensor (in which the temperature of a heating body that generates heat due to current flowing therethrough is measured using a temperature sensing element) may be configured in which heat conduction from the heating body to the temperature sensing element is influenced by a surrounding liquid. In this concentration sensor, a measured temperature of the heating body reflects the concentration of the liquid. Therefore, the urea concentration of a urea aqueous solution can be detected according to a relationship between the urea concentration and the temperature variation of the heating body. This is done by energizing the heating body for a prescribed time and measuring the temperature of the heating body with the temperature sensing element before and after energization (refer to JP-A-2005-84026, for example). The relationship between the urea concentration and the temperature variation of the heating body also depends on the (initial) liquid temperature. Therefore, in JP-A-2005-84026, the urea concentration of a urea aqueous solution is detected by measuring both the temperature of the urea aqueous solution using a temperature sensing element different from the one used to measure the temperature variation of the heating body. JP-A-2005-84026 proposes to issue an alarm on the basis of output information of the separate temperature sensing element, if the temperature of a urea aqueous solution is detected as being as low as its freezing point.
3. Problems to be Solved by the Invention
However, the concentration sensor of JP-A-2005-84026, whose concentration sensing portion is provided with a temperature sensing element separate from that accompanying the heating body, is disadvantageous in that the concentration sensor is large and the circuitry for the concentration detection has a complex configuration.
Furthermore, a urea aqueous solution contained in the urea solution tank may freeze in colder climates. In such a case, the urea aqueous solution cannot be jetted onto the catalyst, and hence it is necessary to wait until it melts. As described above, although JP-A-2005-84026 issues an alarm which notifies of freezing of a urea aqueous solution using the temperature sensing element that is separate from the one accompanying the heating body, it does not refer to any processing to be performed on the heating body when the urea aqueous solution is frozen. The concentration sensing portion may be damaged if the step of detecting urea concentration by energizing the heating body for a prescribed time is repeated while the urea aqueous solution is frozen. More specifically, if the heating body is energized for a prescribed time in a state such that the urea aqueous solution is frozen, part of the urea aqueous solution around the concentration sensing portion melts due to the generated heat. However, the melted portion of the urea aqueous solution is frozen again if most of the urea aqueous solution remains frozen, and the concentration sensing portion may be damaged due to pressure caused by freezing expansion.